Most vehicles in operation today are powered by internal combustion (IC) engines. Internal combustion engines typically have multiple cylinders or other working chambers where combustion occurs. The power generated by the engine depends on the amount of fuel and air that is delivered to each working chamber.
The combustion process and the firing of cylinders can introduce unwanted noise, vibration and harshness (NVH). For example, the engine can transfer vibration to the body of the vehicle, where it may be perceived by vehicle occupants. Sounds may also be transmitted through the chassis into the cabin of the vehicle. Under certain operating conditions, the firing of cylinders generates undesirable acoustic effects through the exhaust system and tailpipe. Vehicle occupants may thus experience undesirable NVH from structurally transmitted vibrations or sounds transmitted through the air.
There are a wide variety of ways to improve the acoustic and vibration characteristics of a vehicle. Typically, vehicles utilize engine mounts that both support the engine and absorb vibration from the engine. In some vehicles, the engine mount is active e.g., it can be stiffened or made more compliant depending on the engine speed and other conditions. For example, when the engine is at idle or under low load conditions, the active mount may become more compliant so that the vibration is better absorbed. At higher speeds, however, the mount may be stiffened to prevent excessive engine motion from damaging the connections between the engine and its attached components.
Some vehicles use a passive exhaust valve to help reduce engine noise. For example, the exhaust valve may involve a flap that is situated near the tailpipe along a line that connects the exhaust ports of the cylinders to the tailpipe. The flapper valve impedes the exhaust flow from the cylinders to the tailpipe. If the exhaust flow rate is low, the flap may tend to close, while high exhaust flow rates force the flap to open more widely. The flapper valve helps to dampen, reflect, or modulate pressure waves in the exhaust path that are generated by the engine, thereby reducing undesirable acoustic effects.
To help improve passenger comfort and reduce undesirable sounds in the cabin of a vehicle, an active noise cancellation system may be used. In some vehicles, for example, there are one or more speakers and microphones situated within the cabin. When noises from the road, engine or other parts of the vehicle enter the cabin, the microphones detect the noise. The noise is analyzed and used to generate canceling sounds through the speakers. The amplitude, phase, frequency and wavelength of the generated sound waves are selected to cancel the undesirable acoustic effects.
Fuel efficiency of many types of internal combustion engines can be substantially improved by varying the displacement of the engine. This allows for the full torque to be available when required, yet can significantly reduce pumping losses and improve thermodynamic efficiency through the use of a smaller displacement when full torque is not required. The most common method of varying the displacement today is deactivating a group of cylinders substantially simultaneously. In this approach no fuel is delivered to the deactivated cylinders and their associated intake and exhaust valves are kept closed as long as the cylinders remain deactivated.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip fire” engine control. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, a particular cylinder may be fired during one engine cycle and then may be skipped during the next engine cycle and then selectively skipped or fired during the next. Skip fire engine operation is distinguished from conventional variable displacement engine control in which a designated set of cylinders are deactivated substantially simultaneously and remain deactivated as long as the engine remains in the same variable displacement mode. Thus, the sequence of specific cylinders firings will always be exactly the same for each engine cycle during operation in a variable displacement mode (so long as the engine remains in the same displacement mode), whereas that is often not the case during skip fire operation. For example, an 8 cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it is operating using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three fixed displacement modes.
In general, skip fire engine operation facilitates finer control of the effective engine displacement than is possible using a conventional variable displacement approach. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. Conceptually, virtually any effective displacement can be obtained using skip fire control, although in practice most implementations restrict operation to a set of available firing fractions, sequences or patterns.
Many skip fire controllers are arranged to provide a set of available firing patterns, sequences or firing fractions. In some circumstances the set of available firing patterns or fractions will vary as a function of various operating parameters such as engine load, engine speed and transmission gear. Typically the available firing patterns are selected, in part, based on their NVH characteristics. Transitions between firing fraction levels must be managed to avoid unacceptable NVH during the transition. In particular, changes in the firing fraction must be coordinated with other engine actuators to achieve smooth firing fraction transitions.
The Applicant, Tula Technology, Inc., has filed a number of patents describing various approaches to skip fire control. By way of example, U.S. Pat. Nos. 8,099,224; 8,464,690; 8,651,091; 8,839,766; 8,869,773; 9,020,735; 9,086,020; 9,120,478; 9,175,613; 9,200,575; 9,200,587; 9,291,106; 9,399,964, and others describe a variety of engine controllers that make it practical to operate a wide variety of internal combustion engines in a dynamic skip fire operational mode. Each of these patents and patent applications is incorporated herein by reference.
In some applications referred to as multi-level skip fire, individual working cycles that are fired may be purposely operated at different cylinder outputs levels—that is, using purposefully different air charge and corresponding fueling levels. By way of example, U.S. Pat. No. 9,399,964 (which is incorporated herein by reference) describes some such approaches. The individual cylinder control concepts used in dynamic skip fire can also be applied to dynamic multi-charge level engine operation in which all cylinders are fired, but individual working cycles are purposely operated at different cylinder output levels. Dynamic skip fire and dynamic multi-charge level engine operation may collectively be considered different types of cylinder output level modulation engine operation in which the output of each working cycle (e.g., skip/fire, high/low, skip/high/low, etc.) is dynamically determined during operation of the engine, typically on an individual cylinder working cycle by working cycle (firing opportunity by firing opportunity) basis. Three level (high, low, skip) cylinder output level modulation control may be characterized by a firing fraction (FF), which is the fraction of fired firing opportunities to total firing opportunities, and a level fraction (LF), which is the ratio of high firings to total firings. An effective firing fraction (EFF) can be determined as EFF=FF*LF+FF*R*(1−LF), where R is the ratio of the low firing output to the high firing output.
It should be appreciated that cylinder output level engine operation is different than conventional variable displacement in which when the engine enters a reduced displacement operational state, a defined set of cylinders are operated in generally the same manner until the engine transitions to a different operational state.